1. Field of the Invention
The present invention relates to a metal foil connection of a first and a second metal foil. The first and second metal foils each have a thickness of less than 0.05 mm, and are brazed to one another at a connecting point. The connecting point forms a wedge which is filled with brazing medium. The invention furthermore relates to a honeycomb body of sheet metal layers. The sheet metal layers are formed from metal foils which are at least partly structured and have a thickness of less than 0.05 mm. The sheet metal layers are at least partly brazed to one another. They have one or two respective wedges filled with brazing medium at the brazed connecting points. The invention also relates to a metal foil-brazing medium particle fraction for manufacturing a brazed connection, and a method for manufacturing a metal foil connection of the first and second metal foils through the use of a metal foil-brazing medium particle fraction.
Brazing methods and brazed connections, for example for a metallic honeycomb body, are state of the art for sheet metal layers. German Patent DE 42 19 145 C1, corresponding to U.S. Pat. No. 5,431,330, disclose immersing a honeycomb body in a fluidized bed of brazing powder. The pre-prepared honeycomb body forms brazing medium particles at desired points from a brazing medium particle fraction. The size of the brazing medium particles should be between 1 and 200 micrometers, preferably between 38 and 125 micrometers. Particle sizes in the lower half of that range are more frequently desired than in the upper half. Other methods for applying brazing medium are also disclosed in that document. The methods for applying brazing medium belonging to the prior art are used successfully in brazing honeycomb bodies having sheet metal layers which are made of metal sheets with a material thickness of at least 50 micrometers and more.
It is accordingly an object of the invention to provide a metal foil connection, a honeycomb body, a metal foil brazing medium particle fraction for metal foils and a method for manufacturing a metal foil connection, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which provide a durable metal foil connection for thin metal foils with a thickness of less than 50 micrometers, in particular less than 40 micrometers.
With the foregoing and other objects in view there is provided, in accordance with the invention, a metal foil connection, comprising a first and a second metal foil having a thickness of less than 0.04 mm. The first and the second metal foils are brazed to one another at a connecting point forming a wedge. Brazing medium substantially fills the wedge and has a mass ML. The first and the second metal foils have sections contacted by the brazing medium in the wedge. The sections having a mass MF. The mass ML of the brazing medium and the mass MF of the sections of the metal foils contacted by the brazing medium in the wedge are in a given ratio MF/ML of between substantially 4 and substantially 8.
With the objects of the invention in view, there is also provided a metal foil connection, comprising a first and a second metal foil having a thickness DF of less than 0.04 mm. The first and the second metal foils are brazed to one another at a connecting point forming one or two wedges. Brazing medium fills each wedge and has a mass ML in the wedge. The mass ML of the brazing medium in the wedge and the thickness DF of the metal foils are in a ratio ML/DF of substantially between 8 g/m and 16 g/m.
When attempting to braze thinner metal foils with a material thickness of less than 50 micrometers, in particular when manufacturing a honeycomb body, it was determined that cells simply melt away when the honeycomb body is brought to the brazing temperature. It was also determined that the cells of the honeycomb body were deformed. It is only when an amount of brazing medium applied per connecting point was used in accordance with the rule for adjustment described herein-above with respect to sheet metal thicknesses used heretofore, that the amount of brazing medium defined thereby, and introduced into the wedge, could result on one hand in the metal foil not becoming detached and formation of gaps at the edges being prevented, while on the other hand a durable connection of the brazing points was created.
If a metal foil which is used for a metal foil connection has a metal foil thickness DF of between 0.05 mm and 0.03 mm, the mass of brazing medium ML to be used for the metal foil connection is selected, in an unexpected manner, in an approximately linear dependency with respect to the metal foil thickness DF. The thinner the metal foil thickness DF, the less the mass of the brazing medium ML to be used. An upper limit as well as a lower limit for the mass of brazing medium ML which can still be used can consequently be determined for some metal foil thicknesses DF, and interpolated or extrapolated for other metal foil thicknesses. If a relationship is established between the mass of the brazing medium ML and the metal foil thickness DF, an upper limit in the extent of the ratio of ML/DF=14.6 g/m, with a variation of +5% and xe2x88x925% has proved advantageous. A ratio of ML/DF=8.7 g/m, with a variation of +5% and xe2x88x925%, has proved advantageous as a lower limit for the still useable ratio of the mass of brazing medium ML to the metal foil thickness DF. The range to be used for a metal foil thickness DF of less than 0.05 mm to approximately 0.03 mm, can be very accurately determined from these two relationships, given as the upper limit and lower limit. The best results with respect to the durability of the metal foil connection have been produced when a ratio of the mass of the brazing medium ML in the wedge compared to the metal foil thickness DF is approximately ML/DF=11 g/m, with a variation of +15% and xe2x88x9210%.
When using metal foil thicknesses DF of approximately 0.03 mm or less for the metal foil connection, the linear relationship described hereinabove can also be used in order to obtain satisfactory results. However, in an unexpected manner, it has been shown that with metal foil thicknesses DF of less than 0.03 mm, it is not only a linear relationship which exists between the amount of brazing medium that can be used and the metal foil thickness DF. Instead, the gradient of this linearity changes with respect to a range of the metal foil thickness DF of less than 0.05 mm to approximately 0.03 mm. It flattens out somewhat. Preferably, with metal foil thicknesses DF of approximately, or less than, 0.03 mm, an upper limit of the mass of brazing medium ML is selected in dependence upon the metal foil thickness DF along a curve which passes through the following points with coordinates (ML/DF; DF): (14.6 g/m; 0.03 mm), (14.8 g/m; 0.025 mm), (16 g/m; 0.02 mm), (27 g/m; 0.01 mm). A lower limit with a metal foil thickness DF of approximately, or less than, 0.03 mm for the mass of brazing medium ML to be used, in dependence on the metal foil thickness DF, is advantageously selected from a curve which passes along the following points with coordinates (ML/DF; DF): (8.6 g/m; 0.03 mm), (9 g/m; 0.025 mm), (9.2 g/m; 0.02 mm), 16 g/m; 0.01 mm). Extremely durable metal foil connections with a metal foil thickness DF of approximately, or less than, 0.03 mm have been produced when the mass of brazing medium ML in dependence on the metal foil thickness DF is selected from a curve which passes through the following points with coordinates (ML/DF; DF): (11 g/m; 0.03 mm), (11.2 g/m; 0.025 mm), (12 g/m; 0.02 mm), (20 g/m; 0.01 mm). A variation of +5% and xe2x88x925% is also applicable for these curves.
A preferred area of application for the metal foil connections described hereinabove is honeycomb bodies of sheet metal layers.
With the objects of the invention in view, there is additionally provided a honeycomb body, comprising sheet metal layers formed of at least partly structured metal foils having a thickness of less than 0.04 mm or less than 0.05 mm. The sheet metal layers are at least partly brazed to one another at brazed connecting points. The connecting points each have a metal foil connection with two of the metal foils forming one or two wedges. Brazing medium substantially fills the wedges and has a mass ML. The metal foils have sections contacted by the brazing medium in the wedges. The sections have a mass MF. The mass ML of the brazing medium and the mass MF of the sections of the metal foils contacted by the brazing medium in the wedges are in a given ratio MF/ML of between substantially 4 and substantially 8.
With the objects of the invention in view, there is furthermore provided a honeycomb body, comprising sheet metal layers formed of at least partly structured metal foils having a thickness DF of less than 0.04 mm or less than 0.05 mm. The sheet metal layers are at least partly brazed to one another at brazed connecting points. The connecting points each have a metal foil connection with two of the metal foils forming one or two wedges. Brazing medium fills the wedges and has a mass ML in the wedges. The mass ML of the brazing medium in the wedges and the thickness DF of the metal foils are in a ratio ML/DF of substantially between 8 g/m and 16 g/m.
When using the rules for adjustment set out hereinabove for metal foil connections, it has been shown that the durability of the honeycomb body with respect to mechanical stresses was very much higher as compared to when using amounts of brazing medium which were previously standard. When using the most widely differing metal foil thicknesses, the most advantageous amount of brazing medium could be found in a more rapid and simple manner by taking into account the rule of adjustment between the mass of the brazing medium ML and the metal foil thickness DF. However, not only the durability but also the problems of cell burning, cell deformation, destruction of layers and gap formation at the edges described hereinabove were avoided by observing the rules for adjustment for the metal foil connections.
A further procedure for being able to manufacture a durable metal foil connection is obtained by using a suitable metal foil-brazing medium particle fraction.
Therefore, with the objects of the invention in view, there is furthermore provided a metal foil-brazing medium particle fraction in a brazed connection between first and second metal foils forming a wedge, in particular for manufacturing a brazed connection in a honeycomb body formed of metal foil, comprising a particle size between 0.001 mm or 0.01 mm and 0.2 mm. A maximum diameter of 0.135 mm and a minimum diameter of 0.015 mm are provided for a metal foil thickness of substantially 0.05 mm. A maximum diameter of 0.08 mm and a minimum diameter of 0.02 mm are provided for a metal foil thickness of substantially 0.02 mm. A substantially linear maximum diameter and a substantially linear minimum diameter are provided for a metal foil thickness between substantially 0.05 mm and substantially 0.02 mm. A maximum value of a Gaussian distribution in percent is provided for a respective portion of the diameter disposed substantially centrally between the maximum and the minimum diameters.
In an unexpected manner, a linear relationship has been found between the maximum diameter and minimum diameter of the brazing medium particles of a brazing medium particle fraction for the respective metal foil thicknesses to be connected. Furthermore, very durable metal foil connections for metal foil thicknesses DF of approximately, or less than, 0.05 mm, in particular 0.03 mm or less, have been obtained, in that the maximum value of the Gaussian distribution is not displaced towards a smaller brazing medium particle fraction as the metal foil thicknesses become less, but instead remains disposed in the center within the distribution. An extremely durable metal foil connection resulted when the bell-shape of the Gaussian distribution was retained in the center with decreasing metal foil thicknesses, and when it did not change with metal foil thicknesses which were up to 0.01 mm.
In the case of a maximum diameter of the brazing medium particle fraction, the following rule for adjustment has proved extremely advantageous: the maximum diameter of the brazing medium particle fraction results from the following values:
brazing medium particles with a maximum diameter of 0.125 mm and particularly 0.105 mm, for a thickness of approximately 0.05 mm;
brazing medium particles with a maximum diameter of 0.07 mm and particularly 0.063 mm, for a thickness of approximately 0.02 mm; and
a maximum diameter of the brazing medium particles, which is produced in an approximately linear manner from the corresponding values for the thickness of the metal foil of 0.05 mm and 0.02 mm, for a thickness of metal foil which lies therebetween.
In the case of a minimum diameter of the brazing medium particle fraction, the following rule for adjustment proved extremely advantageous: the minimum diameter of the brazing medium particle fraction results from the following values:
brazing medium particles with a minimum diameter of 0.018 mm, in particular 0.023 mm, for a thickness of approximately 0.05 mm;
brazing medium particles with a minimum diameter of 0.03 mm, in particular 0.035 mm, for a thickness of approximately 0.02 mm and
a maximum diameter of the brazing medium particles, which is produced in an approximately linear manner from the corresponding values for the thickness of the metal foil of 0.05 mm and 0.02 mm, for a thickness of metal foil which lies therebetween.
In the case of a thickness of the metal foil of 0.03 mm or less, it was determined, in an unexpected manner, that the minimum diameter of the brazing medium particles should not decrease. Rather, the metal foil connections were particularly durable when the minimum diameter was approximately 0.03 mm, in particular 0.035 mm. Brazing medium particles with a diameter less than that did not increase durability. Rather, a deterioration was frequently determined.
With the objects of the invention in view, there is also provided a method for manufacturing a metal foil connection of first and second metal foils using a metal foil-brazing medium particle fraction, in particular for a honeycomb body formed of metal foil, which comprises providing the first and second metal foils with a thickness of less than 0.05 mm; applying glue to the first and second metal foils; subsequently placing the metal foil-brazing medium particle fraction in contact with the first and second metal foils; and brazing the first and second metal foils together at a durable connecting point forming one or two wedges. The metal foil-brazing medium particle fraction is provided with a particle size between 0.001 mm and 0.2 mm; a maximum diameter of 0.135 mm and a minimum diameter of 0.015 mm for a metal foil thickness of substantially 0.05 mm; a maximum diameter of 0.08 mm and a minimum diameter of 0.02 mm for a metal foil thickness of substantially 0.02 mm; a substantially linear maximum diameter and a substantially linear minimum diameter for a metal foil thickness between substantially 0.05 mm and substantially 0.02 mm; and a maximum value of a Gaussian distribution in percent for a respective portion of the diameter disposed substantially centrally between the maximum and the minimum diameters.
With the objects of the invention in view, there is additionally provided a method for manufacturing a metal foil connection of first and second metal foils using metal foil-brazing medium particle fractions. The method comprises providing the first and second metal foils with a thickness of at most 0.03 mm and applying glue to the first and second metal foils. The first and second metal foils are subsequently contacted with a first metal foil-brazing medium particle fraction in a first step. The first and second metal foils are subsequently again contacted with a metal foil-brazing medium particle fraction in a second step. The first and second metal foils are brazed together at a connecting point forming wedges.
In an unexpected manner, a two-step method of application of brazing medium has proved more advantageous with such material thicknesses of the metal foil than a single-step application of brazing medium, despite the high cost. A better durability was obtained, as well as a better control of the amount of brazing medium being introduced, as compared to a purely single-step application of the metal foil-brazing medium particle fraction.
This two-step method is further improved by selecting the first metal foil-brazing medium particle fraction in such a way that it has a greater maximum and a smaller minimum diameter of the brazing medium particles than a metal foil-brazing medium particle fraction used in the second step. Advantageously, in the first step, the first metal foil-brazing medium particle fraction is adjusted as has previously been described hereinabove. The second metal foil-brazing medium particle fraction is again advantageously selected for the second step in such a way that the maximum diameter of the brazing medium particles is less than 0.07 mm and the minimum diameter of the brazing medium particles is greater than 0.04 mm. When these rules for adjustment are observed, particularly durable metal foil connections are produced. In particular, with the brazing of a honeycomb body, the metal foil connections were produced with an extremely low or no failure rate, directly after the brazing procedure, as well as in subsequent tests.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a metal foil connection, a honeycomb body, a metal foil brazing medium particle fraction for metal foils and a method for manufacturing a metal foil connection, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.